Oscillatoriacece 329 



There are three British species of the genus occurring in fresh water (or 

 sometimes in slightly brackish water). None of them are abundant, although 

 N. spumigena Mertens is not uncommon (diameter of trichomes 10 15 p,). 

 N. sphcerocarpa Born. & Flah. is known from Cambridgeshire (diameter of 

 trichomes 67 /* ; fig. 150 H). 



Genus Cylindrospermum Ktitz., 1843. The trichomes in this 

 genus are relatively short, destitute of a sheath, and are mostly 

 aggregated to form an expanded mucous stratum. The cells are 

 cylindrical, longer than their diameter, and one end of the trichome 

 is terminated by a solitary heterocyst. The spores are always 

 developed from the cell (or cells) next the heterocyst; they are 

 generally solitary, but in one species they are seriate. The terminal 

 position of the heterocysts and spores at once distinguishes this 

 genus from all the others of the Nostocacese. 



There are three British species. C. majus Kiitz. ( = C. macrospermum 

 Rabenh.) is common on damp stones and earth, often forming a thin stratum 

 on garden paths or at the edge of a pond ; thickness of trichomes 5 6 /*. 

 C. stagnate (Kiitz.) Born. & Flah. is not uncommon in boggy ditches, particu- 

 larly in peaty ditches; thickness of trichomes 3 - 8 4'5/x; fig. 150 E G. 

 C. catenatum Ralfs is a much rarer species occurring on the damp mud of 

 stagnant rivers and ditches. 



Family 4. OSCILLATORIACE^l. 



This is the largest family of the Psilonemateae and is 

 distinguished at once from all the others by the entire absence 

 of heterocysts. The trichomes consist of a simple and uniform 

 row of cells, which sometimes exhibits a slight attenuation at the 

 apex. The apical cell may be rounded, conical, or subcapitate, and 

 it occasionally carries a slightly thickened hood or calyptra. The 

 trichomes are straight or flexuose, occurring either free-floating or 

 forming a thin mucous stratum. The slimy stratum formed by 

 filaments of this family may occur on damp surfaces, on submerged 

 stones, or on the muddy bottom of ponds and ditches. Some 

 species of Oscillatoria rise to the surfaces of ponds as brown or 

 brownish-green flocculent masses, which are often very conspicuous. 

 This generally occurs in bright sunshine, and the mass floats owing 

 to the retention of numerous bubbles of oxygen amongst the 

 intricate network of trichomes. The cells of some species of this 

 family are joined so closely that the trichome appears perfectly 

 cylindrical with straight margins. In other species there is a faint 



